U.S. Pat. No. 4,496,817 describes a cooking process that is divided into three phases, namely a normal cooking phase, a drying out phase, and a pre-combustion phase. At the beginning of the normal cooking phase, the concentration of organic gases, such as carbon dioxide or carbon monoxide, in the cooking chamber is relatively low. As the cooking chamber continues to be heated, the concentration of organic gases increases rapidly. Normal cooking processes should be terminated prior to reaching this point.
At times, this point may be exceeded, for example, due to user errors, so that the cooking chamber continues to be heated. As a result, the temperature of the food load in the cooking chamber continues to rise while the gas concentration increases only very slowly. This phase is referred to as the drying out phase. Before the temperature of the food load reaches a point where the food load ignites, the food begins to char, resulting in another sharp increase in the concentration of organic gases in the cooking chamber. This phase is referred to as the pre-combustion phase.
To ensure safety in the event of a user error, U.S. Pat. No. 4,496,817 describes a method in which in order to distinguish the individual phases, the system measures the concentration of an organic gas, such as carbon dioxide or carbon monoxide, in the cooking chamber.
Once the gas concentration exceeds a predetermined threshold, the controller of the baking oven detects that the normal cooking phase has ended, and that now the current cooking process is in the so-called drying out phase. After that, the rate of change of the gas concentration is monitored and compared to predetermined limit values in an evaluation circuit of the controller. If the rate of change falls below a lower limit, then the controller detects a transition from the normal and desired cooling at the end of the normal cooking phase to the drying out phase. If subsequently the rate of change of the gas concentration increases sharply again and exceeds an upper limit, then the system detects the beginning of the pre-combustion phase.
In order to prevent flaming in the cooking chamber, and thus to prevent a fire condition, the U.S. Pat. No. 4,496,817 proposes that a magnetron for heating the cooking chamber and a blower for circulating air through the cooking chamber be switched off. It is also proposed to generate an audible alarm signal.
U.S. Pat. No. 4,954,694 describes a method for controlling a pyrolytic cleaning process, in which the oxygen concentration in the cooking chamber is measured by an oxygen sensor, and the rate of change of the measured oxygen concentration is evaluated. The overall heating time required is determined from the curve of the rate of change. However, it does not describe the detection of a fire condition.
U.S. Pat. No. 4,481,404 and U.S. Patent Application No. 2002/0014480 each describe a method for pyrolytic cleaning, but do not disclose any fire condition detection either.
Furthermore, U.S. Patent Application No. 2001/0052852 generally describes the control of cooking processes as a function of the concentration of smoke or gas.
Finally, German Patent Application No. DE 103 27 861 A1 describes the use of a cooking quotient for controlling a cooking process, the cooking quotient being calculated from a current rate of change of the oxygen concentration and a first extreme value determined for the rate of change. However, it does not describe the detection of a fire condition.